So? T.s. concinnus (true concinnus) are only found in SW WA and NW Oregon. CA red-sided garters are also limited to CA. Plenty of both snakes available.
Since concinnus are native to other states, it's no surprise there's plenty in the hobby. The only CA red-sideds I've seen for sale have been over $100. I consider that a pretty high price for a snake. How many hobbyists in the U.S. who keep garters do you think have CA red-sideds (as oppposed to easterns, checkereds, etc.)?

The fact of the matter is most garters endemic to CA are not commonly found in the hobby (at least in the United States) - I listed three examples - there's more that can be added to the list.

Tim